This invention relates to an exhaust gas purifying system for automobile engines or their likes, and more particularly to an improved purifying system of the type of injecting a secondary air in the vicinity of the exhaust valve for burning the unburned part of the exhaust gases produced in the engine.
According to the conventional exhaust gas purifying system of similar type, there is provided in an air supply conduit an air by-pass valve which is actuated in response to increases in the suction vacuum of an engine as the engine is rapidly decreasing its speed to discharge the air to the atmosphere and temporarily cut off the injection of the air or an anti-afterburn valve which is then actuated temporarily to introduce the air into a suction system of the engine. These valves are intended to prevent any occurrence of after-burning and resultant overheat in an exhaust system due to the effect of reburning large quantities of unburned part of the exhaust gases produced as the engine is rapidly reducing its speed.
Generally, high-temperature gases are exhausted from the engine which is running at high speeds, and contain a small amount of the unburned part therein. Therefore, there is little need of injecting the secondary air for reburning the unburned part. There may also occur overheat in the exhaust system when excess air (secondary air) is fed into the exhaust gases under high speed conditions of the engine for reburning the unburned part.
In order to avoid such occurrence of overheat, therefore, it is also necessary to cut off the injection of the secondary air when the engine is accelerating and is running under high loads.
The valves referred to earlier present disadvantages, however, since they are only actuated when the suction vacuum becomes higher as the speed of the engine is rapidly diminishing, so that it is practically difficult or impossible to cut off the injection of the secondary air under the speed-up and high load conditions of the engine where changes in the suction vacuum are small. Particularly, the air by-pass type valve has a drawback in operation that as the secondary air is discharged to the atmosphere, undesirable noises are very likely to occur.